selftests/net: Fix broken test case in psock_fanout

The error return falue form sock_fanout_open is -1, not zero.  One test
case was checking for 0 instead of -1.

Tested: Built and tested in clean client.
Signed-off-by: NMike Maloney <maloney@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/psock_fanout.c

@@ -305,7 +305,7 @@ static void test_unique_fanout_group_ids(void)
 		exit(1);
 	}
 
-	if (sock_fanout_open(PACKET_FANOUT_CPU, first_group_id)) {
+	if (sock_fanout_open(PACKET_FANOUT_CPU, first_group_id) != -1) {
 		fprintf(stderr, "ERROR: joined group with wrong type.\n");
 		exit(1);
 	}